Sulfur in its reduced form plays an important role in plant metabolism, being involved in the biosynthesis of a wide range of primary and secondary sulfur-containing metabolites. In plants, sulfur metabolism includes the uptake of sulfate from the environment, assimilation into organic compounds, and channeling into proteins and secondary substances.
Plants and microorganisms are able to reduce sulfate to sulfide for synthesis of the thiol group of cysteine. The first step is the activation of sulfate by ATP sulfurylase, forming 5′-adenylylsulfate (APS). APS reductase acts upon APS to generate sulfite, and sulfite reductase converts this sulfite to hydrogen sulfide. This hydrogen sulfide provides the thiol group to cysteine, while the carbon portion of cysteine comes from the serine branch of the pathway. In this branch, serine is converted to O-acetylserine by serine acetyltransferase. Cysteine synthase then catalyzes the reaction of O-acetylserine and hydrogen sulfide to form cysteine.
Cystathionine gamma synthase catalyzes the reaction between O-phosphohomoserine and cysteine, wherein the cysteine donates a thiol group to O-phosphohomoserine, thereby forming cystathionine.
Methionine and sulfur-containing vitamins such as biotin or thiamine are essential in human nutrition. Sulfur-mediated functions include electron transport in Fe/S-clusters, structural and regulatory roles via protein disulfide bridges, and catalytic centers. Additionally, secondary sulfur compounds include signaling molecules, anti-carcinogens and atmospheric compounds. See Hell (1997) Planta 202:138.
Often plant protein is deficient in the sulfur amino acids, especially methionine, as well as other essential amino acids such as lysine and tryptophan. As a result, diets must be supplemented with these amino acids in order to provide a balanced diet. A goal of plant breeding has been to increase the amount of sulfur amino acids present in the seed.
A number of methods have been described for increasing sulfur amino acid content of plants. Some of these methods provide for the overexpression of a high methionine seed storage protein, which entails overexpressing the seed storage protein in a transformed plant. Other methods have attempted seed specific expression of synthetic enzymes in the methionine pathway. Still other methods have focused on enzymatic modification of amino acids and capturing these amino acids in transgenic seed storage proteins. However, these methods have met with limited success. There is therefore a need for an effective and direct method of producing significant levels of the sulfur amino acids in plants and plant seeds.